1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image on a sheet or recording medium with heat and a printer, facsimile apparatus or similar full-color or monochromatic image forming apparatus.
2. Description of the Background Art
Generally, an image forming apparatus includes a fixing device including a fixing member accommodating a heat source therein and a pressing member not accommodating it. The heating member and pressing member are configured to fix a toner image formed on a sheet or recording medium with heat and pressure. A current trend in the image forming apparatus art is toward a heat roller or fixing member having a wall thin enough to reduce the warm-up time of the image forming apparatus from the environment standpoint.
More specifically, to reduce the warm-up time, the temperature of the heat roller must be sharply raised to a preselected range of fixing temperature. It is therefore necessary to reduce the thermal capacity of the heat roller to a noticeable degree. However, the temperature of the heat roller, having such small thermal capacity, rapidly drops just after the start of sheet feed because the heat of the heat roller is absorbed by the press roller and sheets sequentially fed, failing to implement desired fixation quality.
In light of the above, it has been customary with an image forming apparatus of the type described to adopt any one of the following schemes. First, the press roller, which absorbs the heat of the heat roller, is implemented as a thin belt or a sponge roller in order to reduce the thermal capacity of the heat roller, thereby preventing the temperature of the heat roller from rapidly dropping. Second, power, or energy, to be applied to the heat source of the heat roller is increased by use of an exclusive power supply. Third, to promote efficient heating of the heat roller, use is made of induction heating available with eddy current to be generated in a conductive material by the electromagnetic induction of an alternating electromagnetic field or a resistance loss of skin current.
However, the conventional schemes described above are not feasible for a high-speed machine needing additional power other than power for fixation. More specifically, such schemes are applicable only to an about fifty-paper machine in which power of PPM (Papers Per Minute) (A4)×20 W is available for heating the heat roller.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-269692, 11-65186, 2001-27872 and 2001-83831.